The invention relates generally to a method for controlling the quality of a magnetizable developer mix for use in an electrophotographic printing machine developer and, more particularly, concerns a method for regenerating the carrier particles of a developer mix such that regenerated carrier particles are stripped of toner component crust during recycling.
According to electrophotographic or electrographic principles typically utilized in non-mechanical copying or printing machines, electrostatic latent images of characters to be printed are generated on a recording medium, such as a photoconductive drum or specially treated paper. These electrostatic images are subsequently inked with a print powder, called toner particles and which is typically black, in a developer station. The developer station typically includes at least one developer unit generally referred to in the art as a magnetic brush developer. A magnetic brush developer, as a rule, utilizes a two-component magnetizable developer mix for developing the latent charge images recorded on the recording medium. The two components of the developer mix consist of ferro-magnetic carrier particles and toner particles which contain synthetic components. The carrier particles are covered with a very thin, adhering synthetic or oxide layer, which, in the tribo-electric voltage series, lies far apart from the synethic component of the toner so as to produce mutual forces of attraction. A magnetic brush arrangement serves to conduct developer mix particles past the charge images formed on the recording medium, whereupon the toner adheres to the charge images as a result of electrostatic forces.
The concentration of toner particles within the developer mix, i.e., the percentage of toner particles relative to carrier, relates directly to the characteristics of the developed images. For example, the density of the image will be affected by the toner particle concentration. Undesirable decrease in toner concentration in the developer is prevented by means for periodically adding new toner particles to the developer mix. One system adapted for regulating toner powder concentration within a particulate mixture utilized in a developer is disclosed, for example, in U.S. Pat. No. 3,801,196. There, toner powder concentration is measured as a function of the intensity of light rays transmitted by a reflector means having toner powder adhering thereto.
Due to the constant intermingling of carrier and toner particles in a developer mix, a layer of synthetic material, which is a component of the toner, becomes deposited on the synthetic or oxide layer of the carrier particles. This synthetic material causes a crust to form on the carrier particles, the thickness of which becomes greater and greater with time such that the tribo-electrical forces between the carrier and toner particles are gradually reduced. This reduction of the tribo-electrical forces may eventually render the developer mix unuseable, requiring complete replacement of the mix. In the case of fast copiers or printers, this crusting process necessitates that developer mix be replaced often and in very short time intervals in order to maintain suitable printing or copying quality. The frequent replacement of the developer mix causes frequent and long down times of the printer or copier. Furthermore, as the crusting process occurs, printing quality becomes reduced in direct correlation with the increasing age of the developer mix. In addition, since conventional monitoring devices for measuring toner concentration in the developer mix do not discount or make allowance for the crust component of the carrier particles, inaccurate toner concentration measurements result and frequently indicate toner concentration actually increasing with the age of the developer mix. For example, the crust component in old developer mix can amount to 60-80 percent of the measured toner concentration.
An object of the present invention is to eliminate the need for frequent replacement of developer mix in a developer unit and, at the same time, substantially maintain the printing or copying quality of character images being developed.